The present invention relates to a swash plate type variable displacement compressor including a rotary shaft, a swash plate and a plurality of pistons, wherein the swash plate is rotated by driving force of the rotary shaft, the swash plate being inclinable at a variable inclination angle, the pistons being engaged with the swash plate and reciprocable in accordance with the rotation of the swash plate so that the length of the stroke of each piston is varied depending on the inclination angle of the swash plate. The present invention also relates to a method of controlling a solenoid of the swash plate type variable displacement compressor.
In the swash plate type variable displacement compressor, when the swash plate is rotated by the rotation of the rotary shaft, the rotation of the swash plate is transmitted to the pistons via pairs of shoes thereby to cause the reciprocating motion of the pistons for compressing a refrigerant. When the inclination angle of the swash plate is changed with respect to the rotary shaft, the length of the stroke of each piston is changed thereby to vary the displacement of the swash plate type variable displacement compressor.
The swash plate type variable displacement compressor which is disclosed by Japanese Unexamined Patent Application Publication No. 2007-24257 has an electromagnetic clutch as the power transmission mechanism between the rotary shaft and the engine. The electromagnetic clutch is provided outside the compressor housing of the swash plate type variable displacement compressor. If the electromagnetic clutch is not used as the power transmission mechanism, power produced by the engine may be transmitted to the rotary shaft at all times. In such a swash plate type variable displacement compressor having no electromagnetic clutch, the engine rotates the rotary shaft constantly. In a vehicle air conditioner, therefore, when cooling operation is not needed, the displacement of the swash plate type variable displacement compressor is minimized by keeping the swash plate at the minimum inclination angle position. The minimization of the displacement reduces the load applied to the engine thereby to improve the fuel efficiency of the engine.
In the swash plate type variable displacement compressor with or without the electromagnetic clutch has pairs of shoes which are disposed in sliding contact with the swash plate. The sliding resistance between the shoes and the swash plate causes a mechanical loss, thereby providing the additional load applied to the engine. Particularly in a swash plate type variable displacement compressor having no clutch, the mechanical loss caused by the sliding resistance needs to be reduced in order to reduce the load applied to the engine when the compressor is operated at its minimum displacement (or with the swash plate placed at the minimum inclination angle position).
In the swash plate type variable displacement compressor which is disclosed by Japanese Unexamined Patent Application Publication No. 2006-152918, the swash plate is supported by its support that rotates integrally with the rotary shaft. The swash plate and the support are connectable to and disconnectable from each other via a clutch. The clutch is operable between a first state (or the engaged state) where the swash plate and the support are rotated integrally and a second state (or the disengaged state) where the swash plate is rotatable relative to the support. The spring force of a compression spring provided in the support and the centrifugal force acting on a spherical body provided between the swash plate and the support urge the swash plate in the direction that causes the power transmitting portion of the support and the power receiving portion of the swash plate to be disengaged from each other. By so constructing the compressor, shifting may be done between the first state of the clutch where the priority is given to the improvement of the displacement controllability when the swash plate is placed other than at the minimum inclination angle position, and the second state of the clutch where the priority is given to the reduction of the rotational resistance when the swash plate is placed at the minimum inclination angle position.
If the power transmitting portion of the support and the power receiving portion of the swash plate are disengaged from each other when the swash plate is placed at the minimum inclination angle position, the above-described problem raised by the swash plate type variable displacement compressor having no clutch is resolved. In the case of the swash plate type variable displacement compressor having the electromagnetic clutch, the disadvantage of a large power consumption due to energization of the electromagnetic clutch is also avoided.
However, the load that urges the swash plate toward the support when the swash plate is placed at the minimum inclination angle position depends on the rotational speed of the rotary shaft. The urging load is reduced and then increased with an increase of the rotational speed of the rotary shaft. For allowing the clutch to be shifted from the second state (or the disengaged state) where the swash plate and the support are disconnected from each other to the first state (or the engaged state) where the swash plate and the support are connected to each other, therefore, the spring load of the compression spring needs to be reduced approximately to the minimum value of the urging load. In the case of a relatively low rotational speed of the rotary shaft or a rotational speed of the compressor during the idling of a vehicle where the centrifugal force acting on the spherical body is relatively small, such a spring load of the compression spring cannot release the clutch from the engaged state, so that the mechanical loss incurred while the rotary shaft is rotating at a relatively low speed, e.g. idling operation of the engine, may not be reduced.
The present invention is directed to providing a swash plate type variable displacement compressor having an electromagnetic clutch that reduces the mechanical loss and the power consumption.